1. Field of the Invention
The present invention relates to a method and apparatus for developing a screen of a cathode ray tube (CRT) and, more particularly, to a method and apparatus for depositing a light-absorptive black matrix material and a light-emitting phosphor material (referred to hereinafter more simply as `screen structure material`) on an inner surface of a panel by using an electrophotographic screening process.
2. Description of the Related Art
Generally, CRTs are designed to reproduce original picture images through exciting phosphors coated on the inner surface of the panel by electron beams emitted from an electron gun.
The screen portion of the panel is formed with a light-absorptive black matrix layer and a light-emitting phosphor layer.
Conventionally, in the process of forming the black matrix and phosphor layers, slurries of screen structure materials and photosensitive binders are deposited on the panel screen portion. And, this is known as a wet photolithographic screening process.
However, the wet process requires numbers of processing steps as well as large quantities of chemicals, causing an environmental pollution.
Therefore, in recent years, an electrophotographic screening (EPS) method, which uses dry-powdered screen structure materials in the screen developing process, has been widely used in substitute for the photolithographic screening method.
The EPS method includes the steps of cleaning the inner surface of the panel, sequentially forming an organic conductive (OC) layer and an organic photoconductive (OPC) layer on the cleaned surface of the panel, establishing a substantially uniform electrostatic charge on the OPC layer, inserting a shadow mask into the panel, exposing selected areas of the OPC layer to visible light to affect the charge thereon, and developing the selected areas of the OPC layer with a screen structure material. The screen structure material is dry-powdered and tribo-electrically charged before the developing operation.
FIG. 5 is a schematic view of a conventional screen developing apparatus for a CRT illustrating the EPS method. As shown in FIG. 5, the developing apparatus includes a chamber 2 having a supporting member 6 for supporting a panel 4, a screen structure material reservoir 12 for storing and feeding the screen structure material, and a spray gun 10 for spraying the screen structure material on an inner surface of the panel 4.
The spray gun 10 is communicated with the screen structure material reservoir 12 through a tube 8.
The screen structure material is mixed with a suitable quantity of air and fed into the spray gun 10 through the tube 8.
In addition, a grid 16 is provided on the supporting member 6 to control the electric field in the vicinity of the OPC layer coated on the inner surface of the panel 4. And a high voltage source 18 is connected to the grid 16 to supply voltage thereto.
Furthermore, the chamber 2 is provided with an exhaust port 20 to remove excess screen structure material that is not deposited on the inner surface of the panel 4.
In the aforementioned screen developing apparatus, the spray gun 10 receives the screen structure materials passing through the tube 8 and sprays it onto the inner surface of the panel 4. At this time, the spraying operation is performed in such a state that the spray gun 10 is vertically spaced apart from the inner surface of the panel 4 and internally fixed on the center of the chamber 2.
However, in the aforementioned spraying operation, the resultant screen layer on the center and corner portions of the panel is not uniform in thickness because the sprayed screen structure material is relatively focused on the portion toward which the spray gun is directed.
As shown in FIG. 6, the dot width d1 of the central portion of the panel is maintained at 150 .mu.m while the dot width d2 of the corner or peripheral portion is maintained at 100.about.150 .mu.m.
Accordingly, difference in the dot width of the screen structure material on the center and corner of the panel causes serious defects in the CRT screen.